The present invention relates generally to improvements in portable telephones and the like. More specifically, the present invention relates to improvements in the acoustic output of narrowband magnetic transducers used in alerters, for such phones and devices, flowing from the use of a phase inverting acoustical enclosure.
Magnetic transducers, devices which convert electrical energy into mechanical energy in the form of sound waves, are typically based on a moving-coil or a moving-armature design. Due to their small size and low cost, moving-armature magnetic transducers often find use in portable cordless or cellular phones as alerters which may also be referred to as ringers or buzzers. Typically, a moving-armature transducer includes a diaphragm which produces sound, the sound being emitted from front and rear holes in the transducer. Unlike moving-coil (dynamic) magnetic transducers found in high fidelity speakers and telephone earpiece receivers, smaller moving-armature magnetic transducers having much stiffer diaphragms are narrowband frequency response devices which typically only operate in the 1800 Hz to 2800 Hz range, rendering them unsuitable for use in speech reproduction. In contrast, a moving-coil magnetic transducer can functions from approximately 300 Hz through 3300 Hz and higher, the frequency range typically used to reproduce the human voice for telephone communications.
Most designers of telephone sets use narrowband magnetic transducers as alerters by merely placing an acoustical output hole in the transducer close to a port in a housing of a telephone handset. This design is haphazard because acoustical leaks can greatly affect the output volume, not only lowering the output volume, but causing great variability in the output volume among individual telephone sets. Sound from the front output hole can leak into the telephone housing so that less sound gets through the telephone housing port and to the listener. Sound output from the back of the diaphragm also escapes from a rear hole in the transducer and, through destructive interference, can cancel sound from the front hole, either within the telephone housing or in the listening space.
A more sophisticated mounting scheme uses a gasket, which is typically soft rubber or closed cell foam, to seal around the front of the narrowband magnetic transducer and prevent the sound from the front hole from leaking into the housing or being canceled by sound from the rear hole. But even in this scheme, the sound from the rear holes is lost in the telephone set or leaks out of openings in the set and partially cancels sound from the front hole within the listening space.
U.S. Pat. No. 5,655,017 discloses a portable telephone with a detachable speaker suitable for voice communication having a moving-coil magnetic transducer based on a bass reflex design. The bass reflex speaker increases the acoustic response of the wideband moving-coil magnetic transducer in the frequency range for voice reproduction in hi-fidelity products and telephone communications. For example, a typical moving-coil loudspeaker, 25 mm in diameter and thus approximately 500 square mm in area, might typically have a resonance frequency around 700 Hz. A successful bass reflex design to extend the response to even lower frequencies would require a rear acoustical enclosure in excess of 50 cubic centimeters (cc). In contrast, a miniature moving-armature transducer, such as might be utilized by ever smaller portable telephone and communicator alerters needs to take up less than half that area and be coupled to a far smaller rear enclosure having a volume of approximately 1 to 10 cc. In combination, the resulting lower mass and lower compliance of the moving-armature transducer""s diaphragm and the enclosure""s acoustical compliance produce resonance frequencies in the neighborhood of 2000 Hz. Thus, these magnetic transducers are typically used in very different applications from those in which moving-coil transducers are used. Existing moving-armature alerter designs suffer from having a low acoustical output level due to their small size, as well as narrowband response at higher frequencies. Because of their inherent low compliance and narrowband response, it was not immediately apparent that a moving-armature mechano-acoustic system could be made to function satisfactorily in a phase-inverting mode, particularly with a miniaturized rear acoustical enclosure of the size allowable given typical design constraints in space restricted applications such as portable phones.
The present invention provides improved acoustical alerting output of a narrowband moving-armature transducer which may be advantageously contained within a telephone housing. As addressed above, presently, sound from the front hole of the transducer is typically directed outside of the housing, providing an audible alerting signal, while sound from the rear holes of the transducer is typically directed into the housing and attenuated or lost. While moving-armature magnetic transducers are reasonably high in output sound pressure level over a narrow frequency band, they could be even more efficient if the sound directed into the housing could be redirected out of housing, in the correct phase, so as to reinforce the sound generated by the front of the diaphragm and associated front port. When used as the alerter in cordless telephones, the primary complaint against moving-armature magnetic transducers is their low acoustic level. Therefore, improvements in the audible acoustic output of these devices would be extremely advantageous.
The present invention provides methods and apparatus for increasing the audible output of narrowband magnetic transducers. As discussed above, the sound output from the rear hole of the narrowband magnetic transducer may be lost in the telephone set or leak out of the housing and partially cancel the sound emitted from the front hole of the transducer. A more efficient implementation of a narrowband magnetic transducer would minimize this interference and use the sound from the rear hole to reinforce the sound emitted from the front hole.
The present invention advantageously utilizes a phase inverting acoustical enclosure contained within the telephone handset to augment the sound output of the front hole of a narrowband magnetic transducer. With the phase inverting acoustical enclosure tuned to a frequency below the diaphragm""s resonance frequency, the front hole output is generally reinforced in the frequency band from below the diaphragm resonance to up through the diaphragm resonance. Thus, the acoustical output increases within a frequency bandwidth that is more advantageous for customer alerting. In addition to the higher output sound pressure level, the widened frequency response is extremely useful to: (1) provide a more pleasant lower-frequency alerting signal, (2) provide an alerting signal not as readily attenuated within a room environment in which a portable telephone may be subject to use, (3) provide an alerting signal more likely to be heard by certain listeners with a particular frequency of hearing loss, and (4) provide an alerting signal comprised of multiple frequency components both to avoid being masked by room noise and to provide for distinctive alerting. Utilizing the present invention, these advantages can be enjoyed without the need to deliver additional power to the magnetic transducer, or use a larger or more expensive magnetic transducer.
In addition to cordless telephone handsets, the present invention""s applicability extends to other devices, such as cellular or wireless mobile phones, or other devices that use a narrowband magnetic transducer in a small volume for providing an alerting signal.
A more complete understanding of the present invention, as well as further features and advantages, will be apparent from the following Detailed Description and the accompanying drawings.